The present disclosure generally relates to extruders used in, for example, plastic forming processes, and more particularly, relates to an improved system and method for removing one or more extrusion screw elements from an extruder screw shaft for cleaning, inspection, repairs, replacement, etc.
Extruders are well known and are used extensively in the plastics industry. For example, extruders can be employed in the plastics industry for producing plastic pellets from raw materials. More particularly, raw materials are loaded into a hopper of an extruder which gravity feeds to an extruder screw (or screws) rotatably mounted in a heated barrel or housing. The extruder housing can form a plurality of zones about the screw, including feed, transition, and metering zones. The feed zone conveys the solid or sometimes half-molten or molten raw material out of a feed throat area to the transition zone, which starts compressing the preheated material. Moreover, the transition zone forces the plastic against the heated barrel and continues the melting process, which is usually completed at the beginning of the metering zone. The metering zone typically conveys the molten plastic to a head and die section of the extruder at uniform rates under high pressure. At a die section, the molten plastic passes through a die where it can be continuously formed into round strands or filaments, for example. The strands or filaments can then be passed through a water cooling bath and subsequently fed to a chopper for processing into pellets. As is well known, the pellets can be provided for a variety of applications. Typically, the pellets are used in an injection molding process wherein they are transformed into molded products or articles at high production rates and with good dimensional accuracy.
Conventional extruder screws typically include a rotatable shaft that carries a plurality of screw elements, which can include conveyor elements and/or kneader elements. The elements received on the shaft within the extruder housing are employed to help work and/or convey the product. Each of the elements typically has a central opening therethrough for being annularly received on the shaft. To securely mount a screw element on the shaft, an interference fit arrangement can be provided between the element and the shaft. Other mounting arrangements include the use of one or more slots and a corresponding key provided between the screw elements and the shaft. For example, corresponding slots can be provided on both the screw element and the shaft and a key can be received in the corresponding slots for securely locking the screw element to the shaft. When setting up an extruder, the screw elements can be slid over the shaft from one end, often referred to as the downstream end, and securely held in place against a collar or flange either secured or integrally formed at the other end of the shaft, commonly referred to as the upstream end. Inner surfaces of the screw elements and outer surfaces of the shaft can additionally be splined or serrated to ensure a good coupling between the screw elements and the shaft.
During operation, the extruder often operates at high temperatures and pressures which sometimes forces molten plastic between the screw elements and the shaft and into the splines meshing the screw elements to the shaft. Undesirably, the screw elements often bond to the shaft due to the close tolerances, high temperatures and/or use of highly filled plastics. This can create a problem when desirable to remove one or more of the screw elements from the shaft. Removal or replacement of screw elements from the shaft, sometimes referred to as screw changes, is typically done from time to time. For example, screw changes may be required when going from one type of product to another. Once removed, the screw elements and/or the shaft can be thoroughly cleaned to remove residual plastic from an earlier batch. The cleaned screw elements or new screw elements can be reinstalled on the screw shaft for a subsequent product run. The need for frequent removal of screw elements from the shaft is a problem that has plagued the extruder industry for a long time.
One type of extruder in which screw elements must be removed from the screw shaft from time to time is a BUSS kneader. In the BUSS kneader, a tip of the shaft screw is first removed prior to removal of any of the screw elements. Then, to remove the screw elements, they are often heated to very high temperatures, such as above 900° F., and forced out by using a sledge hammer and a brass bar (so-called “heat and beat” technique). More particularly, to employ this method, an operator places a brass hammer against the element and another operator hits the brass hammer with a sledge hammer until the screw element starts to axially move along the screw shaft.
This removal process can be unsafe because it may put the operator in a perilous position. For example, the operator may have to get on top of the screw element to bang on the sledge hammer. The operator is more likely to slip and fall because of improper footing in this position, and further because a lot of oil from oil lines of the extruder and powder on the screw could cause the operator to slip. In addition, this method of removing screw elements from the shaft is undesirable due to the damage it can cause to the microstructure and/or the dimensional accuracy of the screw elements and the shaft. The hardened and wear resistant screw shaft and elements are particularly susceptible to cracking and stress fracturing. Also, local overheating, which is sometimes used to assist in screw element removal, can cause deformation of the screw elements. In addition to the safety factors and likelihood of damaging the extruder equipment, there is also a concern for the significant amount of time required for removing the screw elements from an extruder shaft. The lengthy amount of time needed for the typical removal of a screw element from the extruder shaft results in an equal amount of machine downtime or lost productivity.